The invention relates to the field of antenna loops for generating coupling magnetic fields. More particularly, the present invention relates to generating strong coupling magnetic fields between a reader and tag.
Radio frequency identification typically uses a transceiver to drive an antenna that generates a field and sends energy and data to a transponder consisting of a small printed antenna and an integrated circuit which receives the energy that turns on the transponder. The transponder then receives the data and responds by sending back data from stored memory in the transponder. In industry parlance, the transceiver is commonly called a reader and the responding circuit a transponder is commonly called a tag. An article can be tagged with a tag being disposed on the article. The return signal may include an identification of thirty-two bytes in additional to return data.
The transceiver and transponder can function at any desired frequency, but they commonly operate on an assigned frequency of 13.56 MHz. Energy available limits the range to only a few feet, which is in the near field of the antenna. The most basic and common antenna is a single turn loop antenna, tuned to resonance, and with impedance matching to a fifty ohm cable. In the near field, energy is primarily transferred by the magnetic field and the effectiveness of the antenna coupling is describe by analyzing the magnetic field in the near field. The magnetic field from reader must be sufficiently high in strength and must sufficiently extend in range to couple sufficient energy to the tag to power the tag and communicate data from the reader to the tag. The magnetic field from the tag must also be sufficient high in strength and must sufficiently extend in range to couple sufficient energy to the reader for communicating data to the reader. Hence, both the reader and tag have loop antenna for creating the respective coupling magnetic fields. The loop antennas have respective magnetic fields and antenna patterns that have respective pattern orientations which are sensitive to polarization. The pattern orientation between the reader and tag fields affects the amount of coupling, and hence affects the amount of required field strength and range.
The field of a basic loop is as follows, for a square loop, having four legs, horizontal top and bottom, and vertical left and right, described here in words for convenience. A tuning capacitor may be disposed in the top leg and a matching network in the bottom leg to which is connected an RF signal source for generating sinusoidal loop current for generating magnetic fields. By way of example, the magnetic field circles the top leg counter clockwise and circles the bottom leg clockwise, so that the magnetic lines are generated orthogonal to the plane of the loop. The antenna loop is always tuned to resonance so that maximum current exists and hence maximum magnetic filed strength. An array of multiple loops is sometimes used to additively increase the field strength for extending the range between the reader and the tag. An array of two loops is commonly used to extend the range to more than double the field of a single antenna. A common array of two antennas has a field with a strong orthogonal horizontal magnetic field produced between the two antennas.
U.S. Pat. No. 6,166,706, Gallagher, teaches two distal loop antennas with a third overlapping coupled loop used to produce a rotating magnetic field. U.S. Pat. No., 5,103,235, Clemens, teaches a figure eight type of antenna with paired leads that are mutually coupled. The objectives described are to reduce the effects of interference and false alarms and to produce a flatter amplitude response and more linear phase versus frequency. Separate antennas are disadvantageously used for receive and transmit. Clemens teaches a conventional antenna amplitude response. U.S. Pat. No. 5,963,173, Lian, teaches adjacent double loop antenna in a figure eight configuration that is operated inphase or out of phase. Two frequencies are used to produce a field that excites a nonlinear magnetic tag. A compensating tuned loop is used to reduce detuning effects which occur when switching between the two phases. Lian teaches the use of two generator driving respective loops. U.S. Pat. No. 5,602,556, Bowers, teaches the use of various loop configurations of the antenna to produce the desired field, and a larger passive untuned loop surrounding that antenna to effectively cancel far field response as a far field canceling antenna. The canceling antenna uses separate antennas for transmit and receive without impedance compensation of the coupled loops.
One problem of these prior readers and tags is the generation of insufficient field strength over a spatial area and over a desired range from the reader to read a tag from a distal position. Another problem is tag polarization sensitivity. Typically, the tag antenna orientation is unknown. The orientation of the tag loop to the field orientation determines the amount of coupling for sufficient reading. The prior art readers and tags may not read reliably due to insufficient field strength and poor coupling due to unpredictable orientation. In some cases the tag may be stationary. Commonly, however, the tag moves through the field, such as on a conveyor belt. In these tag movement situations, different orientations may prohibit the tag from being read as the tag moves through different parts of the field generated by the reader. It is desirable in the reader to increase the signal strength and varied orientation of the magnetic fields for improved magnetic coupling and reading of the tag.
The prior readers have conventional antenna amplitude responses, as shown in Clemens, that have double peak maxima between which is a minimum. Lian teaches the use of tuning circuits to maximize reader and tag responses. Typically, a 100 pf capacitor in parallel with a 1K-ohm resister functions as a tuning circuit connected in the loop distal the transceiver in combination with a matching circuit connected proximal to the transceiver to be used for tuning single loop reader antennas. Typically, in conventional readers, the transmit carrier at 13.56 MHz is generated to power the tag that sends data. Typically, the tag modulates the carrier received and returns the desired data on upper and lower sidebands. The sidebands are approximately plus and minus 500 KHz from the carrier, and only one sideband is used. The antenna is small compared to wavelength and the radiation resistance is very low and the bandwidth is very narrow. This bandwidth is too narrow to pass the received sidebands, so a loading resistor is incorporated in the matching network to lower the Q and widen the bandwidth. This allows the received sidebands to pass, but absorbs much of the transmitted power, reducing the effective range. The tuning circuit produces a passband with good match at the transmitted carrier with return loss below 20 dB and there is a 2 dB return loss match at the sideband frequency that is adequate for the received sideband signal. The loading resistor provides a sufficiently flat band pass for receiver at the sideband signal. However, much of the transmit energy is lost in the loading resistor in the loop. The tuning resistor decreases the coupling efficiency. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide for generation of magnetic fields for coupling between antenna loops.
Another object of the invention is to provide double loop antennas for generating coupling magnetic fields in two dimensions.
Yet another object of the invention is to provide a biaxial double loop antenna for generating coupling magnetic field in three dimensions.
Still another object of the invention is to provide tuning circuits in double loop antennas for generating coupling magnetic fields in three dimensions.
The invention is directed to a reader having a double loop antenna driven by a single transceiver that is connected between the loops of the double loop antenna. In a first aspect of the invention, the double loop antenna provides both transverse and aligned coupling magnetic fields for improved tag orientation insensitivity in two dimensions, the generating magnetic fields tending to add and cancel for generating transverse and aligned magnetic fields. In a second aspect of the invention, two double loops are disposed in parallel with one loop operated in or out of phase respecting the other so as to generate alternating transverse and aligned magnetic fields for improved tag orientation insensitivity. In a third aspect of the invention, a dual double loop antenna is use for generating transverse, aligned and orthogonal magnetic fields in all three respective dimensions for further improved tag orientation insensitivity. In a fourth aspect of the invention, a compensating circuit is used in combination with the reader loop antenna having a tuning circuit and a matching circuit for generating coupling signals that have improved coupling efficiency with reduced loop loading resistor losses. These and other advantages will become more apparent from the following detailed description of the preferred embodiments.